FIG. 1a diagrammatically illustrates the general construction of a conventional laser as comprising three major components: i--a power supply 10, also called the pumping source; ii--an active medium 12; and iii--an optical cavity 14. The power supply 10 supplies power necessary to `pump` or stimulate the active medium 12 to amplify light passing through it. The optical cavity 14 is usually defined by two end mirrors 15 and 16 which are parallel to each other, with one (e.g., mirror 16) being totally reflective, and the other (e.g., mirror 15) being a partial reflector, or laser beam output coupler. The surfaces of the two mirrors are usually coated with multiple layers of metal and/or dielectric materials, so that mirror 16 provides total reflectivity at one end of the cavity, and mirror 15 provides a predetermined degree of partial reflectivity at the other end of the cavity from which the laser light exits the active medium 12.
The laser's cavity-defining mirrors reflect the laser light back and forth through the active medium so as to amplify the intensity of light within the cavity. That portion of the light which passes through the partial reflector 15 forms an output laser beam 17. The power supply or pumping source may comprise any of a variety of energy sources, such as, but not limited to, flash lamps, other lasers, or electric power supplies that produce current in semiconductor diodes or plasma discharges in a fluid, such as a gas within the optical cavity. The active medium can comprise a gas, a solid, or liquid.
Where the laser architecture is configured as a discharge gas laser structure, it usually requires a continuous gas flow, or frequent gas refills, as the vacuum components release impurity gases, or the discharge induces chemical reactions to change the gas composition. A sealed gas laser, on the other hand, does not require a continuous flow of lasing gas. While it may require gas refills, the interval between refills may vary from hours to many years. However, a sealed gas laser entails more stringent manufacturing conditions in terms of material choice, cleanness, etc. In addition, the laser structure normally incorporates a gas reservoir to increase the amount of gas in order to maintain a long laser lifetime.
In continuous flow gas lasers and lasers that can be sealed for a short time, as from a few days to even a few months, there usually is a pressurized gas tank connected with the laser and containing a large amount of lasing gas. In a continuous flow gas laser system, the gas flow connection is typically made through a small orifice, so that fresh lasing gas may be continuously supplied through the orifice during laser operation.
Where the laser is of the type that is to be sealed for a period of time--hours, days, or even months--the connection with gas tank is isolated by a closed valve during laser operation. Used gas is pumped out at regular intervals, when the laser is not in operation, and fresh lasing gas is refilled into the laser. In a long-term sealed laser, there is usually no pressurized tank accompanying the laser system. Any necessary lasing gas refill is performed at the factory because the intervals between refills are long. It should be pointed out that, in a sealed gas laser structure, the lasing gas supply reservoir and the active gas medium region are openly coupled to each other at all times at equal pressure. The lasing gas supply reservoir's function is to increase the amount of gas of a single gas fill, and hence increase the laser lifetime per gas fill. The pressurized gas tank serves to supply fresh lasing gas continuously or repeatedly to the laser system.
FIGS. 1 and 2 diagrammatically illustrate different configurations of a conventional sealed gas laser. FIG. 1 shows a gas laser structure that does not require fluid cooling, and may comprise a HeNe laser, various ion lasers, conduction or diffusion cooled lasers and optically pumped far infrared lasers. FIG. 2 illustrates a gas laser architecture, such as one using carbon monoxide or carbon dioxide as the active medium, that requires fluid cooling (such as through the flow of water or an antifreeze solution), usually through an arrangement of cooling tubes or jackets 29 closely integrated with the active laser bore and/or the pumping source.
In each of these laser architectures, the active medium of the laser comprises a lasing gas 21 that is present within a central region, channel or bore 23 of an enclosure 25, that also includes a lasing gas supply reservoir 27 surrounding and openly coupled with the central active region bore 23. The lasing gas can be pumped by means of electrical discharge, either longitudinally or transversely, or can be pumped by optical irradiation. Cavity mirrors are shown at 26 and 28.
In a conventional cooled discharge laser architecture, since the lasing gas reservoir is integrated with the active laser medium components, the structure has a relatively complex design and a relatively large cross-sectional dimension. This gives rise to substantial technical complexity in the design of the laser. One problem is the fact that the straightness or linearity of the active laser bore is not easy to maintain when the design is complex. Another problem is that the materials must have closely matched rates of thermal expansion.
In the case of direct current (DC) discharge, the gas discharge electrical impedance of a volume of discharging gas is negatively dynamic, and decreases as the discharge current increases. For a continuous wave (CW) DC discharge, the electrical current must be actively stabilized due to the negative impedance; otherwise a run-away or oscillation of the discharge will occur. This means that the DC power supply must employ a feedback control mechanism to monitor the electric current dynamics. Using this feedback, the power supply is able to quickly adjust the output voltage to reverse at the onset of current run-away. Unfortunately, an electric power supply with feedback and adjustment is relatively difficult to design and expensive to manufacture.
To help stabilize the CW DC discharge, a high resistance ballast resistor can be placed in series with the gas discharge, since the voltage drop across the resistor will reduce the tendency of discharge run-away (or oscillation). For example, when the discharge current is increasing at the same time as the gas discharge impedance is decreasing rapidly, the higher current will cause the voltage drop on the ballast resistor to increase. The voltage on the gas discharge section will then decrease, reversing the increase in current trend. A drawback in using a ballast resistor is the fact that large amount of energy is converted into wasted heat in the ballast resistor. In a short pulsed discharge, run-way or oscillation is not a problem. Run-away or not, each pulse ends very quickly, prior to any damage to the power supply or anything else.
In a pulsed laser, the laser medium is excited by pulsed pumping, so that the laser output will also be pulsed. The laser pulse duration will not necessarily be the same as that of the pumping pulse. In particular, the laser pulse has a minimum duration. This means that the laser output will stay at a constant duration even when the pumping pulse duration is considerably shorter than this minimum laser pulse duration. This minimum intrinsic laser pulse duration is dependent only on characteristics of the laser design, such as the gas pressure, cavity configuration, etc.
A linearly polarized laser beam is needed in many applications. Mechanisms to provide a polarized laser output beam include the use of a Brewster window, a wire Grid, a brazed grating, etc. Each of these mechanisms usually requires the installation of an additional component to constrain the laser beam to be polarized. The additional component adds expense, complexity, and space to the laser device. It also introduces additional loss to laser light amplification, resulting in lower laser output power.
A TEM.sub.00 mode laser produces a single spot laser beam, also called a fundamental Gaussian mode laser beam. This laser mode is frequently desirable because of its high energy concentration, coherence, and stability.